After you have selected "Custom RTMP" a new window will pop-up. Here you need to input your Primcast streaming details from your Dashboard. You can also access these details from the setup e-mail that you will receive after the account has been created.
In the "RTMP URL" field add your RTMP streaming address. For example: rtmp://address.srfms.com:port/live
In the "Stream name" field input your desired stream name, the most commonly used name is "livestream".
Select your encoding settings like audio/video bitrate and codec.